The present invention relates to a thermal printer, and more particularly to a recording sheet cleaning apparatus for a thermal printer.
A thermal printer, such as a thermal line printer, forms an image on a heat-sensitive recording sheet such as heat-sensitive paper, using a thermal head having printing elements (heat generating elements). Generally, the recording sheet is pressed to the thermal head by a platen roller to obtain sufficient heat transmission from the printing elements to the recording sheet.
A problem arises in such a thermal printer in that, if debris has adhered to a printing surface of the recording sheet (i.e., the surface at the thermal head side) prior to the insertion of the recording sheet into the thermal printer the debris may interfere with the printing elements of the thermal head. That is, debris may interfere with the heat transmission from the printing elements to the recording sheet, which creates unintentional non-printed portion.
If where a standard heat-sensitive paper is used as the recording sheet, the paper may deform when pressed between a platen roller and the thermal head, so as to wrap around debris. Since debris is generally below 50 .mu.m, there are only a few cases which results in large non-printing portion that can be recognized with naked eye.
However, when a thermal OHP (over head projector) sheet is used as the recording sheet, since the thermal OHP sheet has a high rigidity, it and does not deform to wrap around debris. Therefore, the area affected by the presence of debris is larger, because heat is prevented from transmitting from the surrounding printing elements. Thus, the presence of debris causes a larger non-printing portion that can be recognized with the naked eye.
In a printer where a pressing force for pressing the recording sheet to the thermal head is relatively small, the above-mentioned non-printed portion becomes more significant. For example, with regard to thermal OHP sheets, debris below 50 .mu.m may cause a wrinkle-shaped non-printed portion having a width of approximately 1 mm (in the feeding direction of the thermal OHP sheet.).